In a typical arrangement, a channel or planar wave guide includes an elongated dielectric core, such as doped silica, deposited on a highly insulating buffer layer, e.g. undoped silica, which itself lies on a suitable elongated substrate, such as silicon. The cladding lies as a final deposited layer covering the length and width of the wave guide core, and also covering adjoining regions of the insulating buffer layer. Such wave guides are known either as channel or planar wave guides, and will be hereinafter referred to as "planar".
Ideally the cladding should conform well to the topography of the wave guide and there should be consistency of the deposited cladding layer especially with regard to refractive index homogeneity throughout its volume, both to provide good performance in the transmission of signals and for longevity. In practice however, the presence of voids, particularly those resulting from a failure to cover adequately the "corners", where the core and buffer meet the cladding, are difficult to avoid and can seriously reduce the production yield when such structures are manufactured commercially.